Early spas were therapeutic facilities. They comprised pools whose water was heated and made to circulate and most were available on some basis to the public. With the availability of suitable plastic materials and small reliable water heaters, relatively low cost private spas were made possible. The first private spas included a simple means for circulating the spa water. Later, the circulation system was modified to return water to the spa in jet outlets placed where spa users could position themselves in the path of the water stream issuing from those outlets. In a further innovation, air was entrained into the returning water stream to increase water turbulence.
While the entrainment of air was successful as an improvement, an arrangement that provides even further enhancement is to have a bubble forming system separate from the water return system. That permits introduction of air at other than the water outlets and it permits inclusion of a number of air outlets greater than the number of water outlets. The systems having been thus separated, the air does more than enhance turbulence. The bubbles of air add to the kinds of sensations a spa user may experience. However, adding a separate air system adds substantially to the cost of the system and to its complexity.